Gereon Niedner-Schatteburg

Reactions of simple hydrocarbons with Nb n : Chemisorption and physisorption on ionized niobium clusters

In a Fourier transform‐ion cyclotron resonance mass spectrometer the gas phase reactivities of niobium clusters Nb+n (n=1–28) with molecular hydrogen, water, methane, ethane, n‐propane, n‐heptane, cyclohexane, acetylene, ethylene, allene, benzene, propene, toluene, xylene, and acetonitrile were investigated under single collision conditions as well as the reactivities of oxidized niobium cluster cations with ethylene and benzene. The reactions of larger clusters with a variety of unsaturated hydrocarbons are believed to proceed via long lived ‘‘physisorbed’’ addition intermediate complexes, which subsequently rearrange to form ‘‘chemisorbed,’’ extensively dehydrogenated final products. The overall reaction seems to proceed with near collision rates, almost independent of cluster size. In some cases also the physisorbed primary products are stabilized and detected. Their yields depend sensitively on the specific nature of the reactant, and on the niobium cluster size n. Fully saturated hydrocarbons unable ...

Protonated water clusters and their black body radiation induced fragmentation

Abstract Protonated water clusters H+(H2O)n, n = 5, …, 65 and their perdeuterated analogues were produced in a discharge sour and stored in an electromagnetic ICR-ion trap under collision-free conditions. The rates of their fragmentation or ‘evaporation’ due to absorption of the 300 K black body background radiation exhibit an overall τ ∼ 1 n dependence. Local deviations of some clusters (e.g. n = 21 or 55) from the overall trend are attributed to their higher stabilities. The fragmentation is modeled by extrapolating macroscopic water droplet evaporation to the microscopic clusters.

A three-dimensional quantum mechanical study of vibrationally resolved charge transfer processes in H++H2 at Ecm= 20 eV

A three‐dimensional quantum mechanical study of vibrational state resolved differential cross sections for the direct inelastic and charge transfer channels of the H++H2 system has been carried out at Ecm =20 eV using the infinite order sudden approximation (IOSA). Steric factors, opacity functions, angular distributions, and integral cross sections are calculated. The integral cross sections are in very good agreement with recent experimental results, whereas the angular distributions agree only partially with the experiments. A further comparison of both the theoretical and experimental results with semi‐classical calculations based on the usual trajectory surface hopping method revealed that the present quantum results provide a better description of the experimental observations. The likely shortcomings of the semiclassical method are discussed.

Methane activation by platinum cluster ions in the gas phase: effects of cluster charge on the Pt4 tetramer

Abstract The reactions of cationic and anionic platinum clusters Pt ± n , n =1−9, with methane CH 4 are investigated under single collision conditions in a Fourier-Transform Ion Cyclotron Resonance Mass Spectrometer. The reaction of the platinum clusters proceeds through the activation of C–H bonds of methane and leads to the subsequent elimination of molecular hydrogen H 2 to form the final metal–carbene complex Pt ± n CH 2 . The cation cluster reactions proceed in general with collision rate whereas the anion cluster reactions are more than an order of magnitude slower. The platinum tetramer anion is unique among all the clusters studied, reacting more efficiently than the corresponding cation. Tentative interpretation in terms of electronic and geometric effects is performed.

High resolution photoelectron spectra of the NO dimer

High resolution zero kinetic energy (ZEKE) photoelectron spectra of the NO dimer are measured. They provide information about the ionization energy of the neutral, as well as about the binding energy, vibrations and structure of the ionized dimer indicating considerable structural reorganization of the dimer upon ionization.

STABILITY AND REACTIVITY OF HYDRATED MAGNESIUM CATIONS

Abstract Unimolecular fragmentation and bimolecular reactions with HCl of water clusters which nominally contain Mg + cations were studied in an FT-ICR spectrometer. A cluster fragmentation and successive evaporation of single water molecules occurring on a millisecond timescale and driven by ambient black body radiation is triggering interesting intracluster reactions. Below a certain critical size (∼17 water molecules) MgOH + forms, and a hydrogen atom is ejected. Similarly bimolecular reactions of Mg aq + clusters with HCl result in a release of H atom and formation of MgCl aq + . Both findings can be rationalized by assuming that the solvated Mg + cations actually detach an additional electron forming a Mg aq 2+ and e aq − within clusters with more than 17 water molecules. Mg + formed by recombination when not enough solvent is available to stabilize the separate charged species then reacts with a water molecule resulting in H-atom formation. Detailed studies of the ion reactions and fragmentation provide additional insights into the structure and stability of solvated magnesium cations.

Methane activation by rhodium cluster argon complexes

Abstract Rhodium cluster argon complexes Rh n + Ar m are produced by laser vaporization followed by supersonic expansion, stored in an FT-ICR mass spectrometer, and their reactions with methane investigated. Ligand exchange reactions are observed, in which up to three argon atoms are replaced by methane. In addition, the solvated rhodium dimer and trimer cations are found to dehydrogenate methane. The efficiency of the dehydrogenation depends on the number of argons, with only the dimer exhibiting this reaction without ligands. This dependence of methane activation on the size of the cluster and number of “solvent” argon atoms is discussed, and compared with heterogenous catalysis on bulk surfaces, where activity and selectivity are controlled by pressure and temperature.

The non-resonant two-photon zero kinetic energy photoelectron spectrum of CS2

Abstract The high-resolution zero kinetic energy photoelectron spectrum of CS 2 is presented. Accurate values for the ionization potential and the spin—orbit splitting of the X + 2 Π g are obtained. Observation of the symmetry-forbidden excitations of the v 2 bending vibration yields accurate frequencies for this normal mode. The Renner—Teller splitting for the Δ u 3 2 and the Σ − u components of v + 2 in the upper 2 Π g 1 2 spin—orbit component has been resolved for the first time. The photoionization efficiency spectra show a strong counter-correlation of the CS + 2 and S + fragments.

Effect of charge upon metal cluster chemistry: Reactions of Nbn and Rhn anions and cations with benzene

The reactions of anionic niobium and rhodium clusters Mn−, M=Nb, Rh, n=3–28, with C6H6 are investigated under single collision conditions in a Fourier-transform ion-cyclotron-resonance mass spectrometer and compared with the results of previous studies on corresponding cationic species. This reveals strong effects of the cluster charge state on hydrocarbon activation as a function of cluster size. Both differences and parallels are observed for reactions of anions and cations. Niobium clusters with a given number of atoms react quite differently than those with a single atom more or less. The fact that almost identical such effects are in the present work found for anion clusters, as for cations with the same number of atoms but two less electrons, suggests that the observed reactivity patterns are more a function of the cluster shape and geometry, than of the details of their electronic structure. The variety of interesting trends and effects observed is interpreted in terms of simple physical models.

Mechanistic Investigation of the Ru-Catalyzed Hydroamidation of Terminal Alkynes

The ruthenium-catalyzed hydroamidation of terminal alkynes has evolved to become a broadly applicable tool for the synthesis of enamides and enimides. Depending on the catalyst system employed, the reaction leads chemo-, regio-, and stereoselectively to a single diastereoisomer. Herein, we present a comprehensive mechanistic study of the ruthenium-catalyzed hydroamidation of terminal alkynes, which includes deuterium-labeling, in situ IR, in situ NMR, and in situ ESI-MS experiments complemented by computational studies. The results support the involvement of ruthenium-hydride and ruthenium-vinylidene species as the key intermediates. They are best explained by a reaction pathway that consists of an oxidative addition of the amide, followed by insertion of a π-coordinated alkyne into a ruthenium-hydride bond, rearrangement to a vinylidene species, nucleophilic attack of the amide, and finally reductive elimination of the product.